Shrinking feature size has been the trend in semiconductor device ever since the advent of integrated circuit (IC) devices. Overall, smaller feature size improves device performance. Examples evidencing this improvement are reflected in the increase of memory device capacity and the increase of the computing power of microprocessors. The push for higher packaging density also leads to 3-dimensional (3D) processing, which place components that used to be located at the top surface of the device inwardly into the body of the device. Examples of such processes are through-silicon-vias (TSVs) and trench metal-oxide-semiconductor field-effect transistors (MOSFETs).
One trench MOSFET design referred to herein as a “planar gate trench MOSFET” has a planar gate structure with polysilicon filled trenches as field plates (sometimes referred to as “RESURF trenches) on both sides of the gate stack, a source in the semiconductor surface between the gate stack and the trenches, and a drain on the bottom surface of the device. For purposes of this patent application, the term “RESURF” is understood to refer to a region/material which reduces an electric field in an adjacent semiconductor region. A RESURF region may be for example a semiconductor region with an opposite conductivity type from the adjacent semiconductor region. RESURF structures are described in Appels, et. al., “Thin Layer High Voltage Devices” Philips J, Res. 35 1-13, 1980.
In contrast to a planar gate trench MOSFET, a trench gate MOSFET, sometimes referred to simply as a trench MOSFET, comprises a trench gate structure that is recessed and perpendicularly oriented relative to the semiconductor surface. A significant feature of trench gate MOSFETs is that it lacks the Junction Field Effect Transistor (JFET) effect.
In the case of a power trench MOSFET, whether it is a planar gate trench MOSFET or a trench gate MOSFET (collectively a “trench MOSFET”), it is common to lay out multiple transistor cells arranged physically and electrically in parallel across the area of the device. Each cell of a trench MOSFET has three separate electrical terminals, the source which is normally shorted to the body, the drain, and the gate. In a switching application, the trench MOSFET operates either in an on-state in which current passes vertically between the source terminal and the drain terminal, or in an off-state in which virtually no current passes between the source and drain terminals. The operation of switching on and switching off for MOSFETs including trench MOSFETs can be modeled by the charging and discharging of a combination of equivalent capacitors between the gate electrode and the body, the source, and the drain. The time it takes to charge and discharge these capacitors determines the switching speed of the MOSFET.